How can some cells become cancer?

Damage to DNA: Every cell has the necessary instructions & plans in it's dna. Cells use this blueprint to carry out all important functions including when to divide & keep growing to when to stop & die. When the dna within the cell gets damaged by the environment (smoking, uv rays, radiation, chemicals etc..) & our cells are not well equipped to repair the damage this leads to errors in the plans causing cancer.

No short answer: Changes to dna is a final common pathway. Things we do (smoke, drink alcohol, obesity) can contribute. Environmental exposures ( toxic waste, radiation, excessive sun exposure) are also causes. Inherited gene mutations ( brca in breast, hnpcc in colon, men in endocrine tumors) confer higher than average risk for those families. Best to avoid bad things, and do screening tests at the right age.

Good question: Cancer describes a range of diseases in which one of your body's cells, or group of cells, undergo a change in genetic background that allows the cell(s) to live and divide regardless of the normal cues that would tell them to stop. This proliferation of careless cells hurts the local environment or that which it spreads to. Changes in dna are caused by environmental exposure plus your biology.

Genes & environment: Cancer arises from genetic changes in cells that lead to their uncontrolled growth. These changes arise from a complex interplay between our inherited genes and our environmental exposures. Hereditary cancers occur when defective genes are inherited, predisposing cells to become malignant. Environmental damage (smoking, hpv, radiation, diet) to cells also can cause these transformations.